1. Field of the Invention
The present invention generally relates to sol-gel mesoporous coatings having a silica based matrix with a low refractive index, which in addition possess antistatic properties as a result of the presence of ammonium groups within the structure thereof, and to methods for manufacturing such coatings. They are mainly intended to be provided on substrates in organic or mineral glass, especially in the ophthalmic optics field.
2. Description of Related Art
Increasingly, the trend is seeking to functionalize items made from mineral or organic glass, by depositing onto the surface thereof coatings that are a few nanometers or micrometers thick in order to impart the same a given property depending on the intended use. Thus, anti-reflection, abrasion-resistant, scratch-resistant, impact-resistant, anti-fogging, anti-fouling or antistatic layers can be provided.
It is well known that optical items, made from substantially insulating materials, have a tendency to build up static electricity charges onto their surface, especially when cleaned under dry conditions by rubbing the surface thereof using a wiping cloth, a piece of synthetic foam or polyester (triboelectricity). Charges that are present on the surface generate an electrostatic field capable of drawing and fixing very low weight-elements that are close at hand (few centimeters), generally small-sized particles such as dust, and this lasts as long as the charge remains on the item.
To reduce or prevent such particle attraction, the electrostatic field intensity should be decreased, that is to say the number of static charges present on the item's surface should be reduced. This may be obtained by making the charges mobile, for example by introducing a layer made of a material inducing amongst the “charge carriers” a high mobility. Materials inducing the highest mobility are conductive materials. Thus, a highly conductive material can disperse charges more rapidly.
The state of the art reveals that an optical item may acquire antistatic properties by incorporating onto its surface, within the functional coating stack, at least one electrically conductive layer, or “antistatic layer”, both expressions being used indifferently.
Such antistatic layer most often may form the outer layer of the functional coating stack, an intermediate layer (inner layer) or be deposited directly onto the optical item substrate.
As used herein, “antistatic” is defined as the ability to retain and/or to develop a substantial electrostatic charge. An item is generally considered as possessing acceptable antistatic properties insofar as it does neither draw nor fix dust or small particles after one of its surfaces has been rubbed using a suitable wiping cloth. It can quickly disperse any accumulated electrostatic charge, so that such an item seems to be “cleaner” after wiping.
The ability for a glass to disperse a static charge resulting from the rubbing using a cloth or any other suitable method for generating an electrostatic charge (through corona discharge, . . . ) may be quantified by measuring the time necessary for said charge to be dispersed. Typically, a glass can be considered as being antistatic when the discharge time is less than or equal to 500 milliseconds. In the present application, a glass is considered as being antistatic when the discharge time thereof is less than or equal to 200 milliseconds.
Known antistatic coatings comprise at least one antistatic agent, which is generally an optionally doped (semi-)conductive metal oxide, such as tin-doped indium oxide (ITO), antimony-doped tin oxide, vanadium pentoxide, or a conjugated structure-conductive polymer.
A number of patent applications (US 2004/0209007, US 2002/0114960 . . . ) describe items provided with an antistatic layer based on conductive polymers deposited directly onto the substrate of the item and independent from the anti-reflection coating. However, conductive polymers are much more expensive than conductive metal oxides. Moreover their presence increases the refractive index and the coating absorption.
The patents EP 0,834,092 and U.S. Pat. No. 6,852,406 describe optical items, especially ophthalmic lenses, provided with an anti-reflection stack which is mineral in nature comprising a transparent, indium-tin oxide-(ITO) or tin oxide-based, antistatic layer, mineral in nature, deposited under vacuum. However, ITO-based antistatic layers do not give fully satisfactory results. As a drawback, they do suffer from non negligibly absorbing in the visible range, so that their thickness has to be relatively low so as not to be detrimental to the transparency properties of an optical item. Moreover, these layers have high refractive indices, generally higher than 1.8.
It would be advisable to provide antistatic coatings possessing lower refractive indices, especially lower than 1.5.
Preparing mesoporous coatings having a silica based matrix possessing a low refractive index due to their high porosity is well known and has been described for instance in the applications WO 2006/021698, WO 2007/088312 and WO 2007/090983, in the applicant's name.
The Japanese applications JP 2008-280193 and JP 2009-040967 describe structured mesoporous coatings with a silica matrix functionalized by NH2 groups, obtained through co-hydrolysis of a tetraalkoxysilane precursor and of an alkoxysilane precursor carrying at least one amino group, typically tetraethoxysilane and 3-aminopropyltriethoxysilane, in the presence of a base and a cationic pore-forming agent selected from surfactants comprising a quaternary ammonium group. The coating may optionally undergo a functionalization reaction of the amino groups before or after extraction of the pore-forming agent, by reacting with an organic compound which carries groups that do react towards the amino groups such as vinyl, carboxy, epoxy or isocyanate groups. The coatings are 50 to 150 nm thick, have a low refractive index (1.33-1.36), and are poorly conductive.
Publications Langmuir 2002, 18, 972-974, J. Sol Gel Sci. Technol. 2007, 43, 305-311, J. Phys. Chem. B 2002, 106, 6652-6658, Chem. Mater. 2008, 20, 4661-4668, Chem. Commun. 2003, 1146-1147 and Chem. Commun. 2004, 1742-1743 describe the preparation of mesoporous coatings having a silica-based- or a polysiloxane based-matrix functionalized by ammonium groups. They are obtained by co-condensating in acidic medium a tetraalkoxysilane- or a bis(trialkoxysilane)-bridged precursor (tetraethoxysilane TEOS or bis-(triethoxysilyl)ethane BTSE) and an alkoxysilane precursor carrying at least one amino or ammonium group (3-aminopropyltriethoxysilane or N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride) and a pore-forming agent, followed with the removal through extraction or thermal treatment of the pore-forming agent. When an amino group-carrying alkoxysilane precursor is used, an acid sufficient amount should be used so as to protonate the amino group and thus to slow down the condensation reaction of the gel-forming silanes. The pore-forming agent is generally a non ionic agent, of the ethylene oxide and propylene oxide block copolymer type or of the polyethylene glycoalkyl monoether type, but it may also be an ionic agent (cetyltrimethylammonium bromide). Both precursors are generally used according to following mole ratio: TEOS to organosilane carrying one ammonium group=1:0.05-0.667.
These mesoporous coatings having a silica based matrix functionalized by amino or ammonium groups as a drawback are sensitive to moisture since water may be easily adsorbed within the pores of the material.